BGP Accumulated IGP

The BGP Accumulated IGP feature is an optional nontransitive Border Gateway Protocol (BGP) path attribute. The attribute type code for the accumulated interior gateway protocol (AIGP) attribute is assigned by the Internet Assigned Numbers Authority (IANA). The value field of the AIGP attribute is defined as a set of type, length, value (TLV) elements. The AIGP TLV contains the AIGP metric.

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Information About BGP Accumulated IGP

Overview of BGP Accumulated IGP

The BGP Accumulated IGP feature is required to simulate the current Open Shortest Path First (OSPF) behavior of computing the distance associated with a path. OSPF or Label Distribution Protocol (LDP) carries the prefix or label information only in the local area. Then, Border Gateway Protocol (BGP) carries the prefix or label to all the remote areas by redistributing the routes into BGP at area boundaries. The routes or labels are then advertised using label-switched paths (LSP). The next-hop for the route is changed at each Area Border Router (ABR) to a local device, which removes the need to leak OSPF routes across area boundaries. The bandwidth available on each of the core links is mapped to the OSPF cost; therefore, it is imperative that BGP carries this cost correctly between each of the provider edge (PE) devices. This functionality is achieved by using the BGP Accumulated IGP feature.

You need to enable accumulated interior gateway protocol (AIGP)
processing for
internal Border Gateway Protocol (iBGP) and external
Border Gateway Protocol (eBGP) neighbors to carry the AIGP
attribute.
Neighbors configured with the AIGP attribute are
put in a separate update group from other iBGP neighbors. A
separate update group is required for neighbors that are enabled to
send the AIGP value to cost community. BGP needs to translate the
AIGP attribute to the cost community or multi-exit discriminator (MED)
and attach it to the route before advertising to legacy.

When BGP installs AIGP attribute routes into the routing information base (RIB), it adds the AIGP cost with the next-hop cost. If the next-hop is a nonrecursive IGP route, BGP sets the AIGP metric to the received AIGP value and the first hop IGP metric to the next-hop. If the next-hop is a recursive route with the AIGP metric, BGP adds the received AIGP metric to the next-hop AIGP metric.

Sending and Receiving BGP Accumulated IGP

When a session receives a prefix with the accumulated interior gateway protocol (AIGP) attribute and is not configured to receive AIGP information, the session discards the AIGP attribute and processes the remainder of the update message, and then it passes the AIGP attribute to other BGP peers. The route is then installed into the routing information base (RIB) and the prefix is sent with the AIGP attribute to all the AIGP-enabled neighbors. The AIGP attribute value is not updated if the next-hop of the route is not changed by the device before advertising it to the neighbor. If the device changes the next-hop of the route, it recalculates the AIGP attribute value by adding the next-hop metric to the received AIGP attribute value.

Originating Prefixes with Accumulated IGP

Origination of routes with the accumulated interior gateway protocol (AIGP) metric is controlled by configuration. AIGP attributes are attached to redistributed routes that satisfy the following conditions:

The protocol redistributing the route is enabled for AIGP.

The route is an interior gateway protocol (IGP) route redistributed into Border Gateway Protocol (BGP). The value assigned to the AIGP attribute is the value of the IGP next-hop to the route or as set by a route policy.

The route is a static route redistributed into BGP. The value assigned is the value of the next-hop to the route or as set by a route policy.

The route is imported into BGP through a network statement. The value assigned is the value of the next-hop to the route or as set by a route policy.

The inbound or outbound route map also creates an AIGP attribute route map using the set aigp-metric command.

How to Configure BGP Accumulated IGP

Configuring AIGP Metric Value

SUMMARY STEPS

1.enable

2.configureterminal

3.routerbgpas-number

4.address-familyipv4[unicast |
multicast |
vrfvrf-name]

5.redistributeprotocolautonomous-system-number
route-map map-tag

6.networknetwork-idroute-map map-tag

7.exit

8.route-map rtmap

9.set aigp-metric [igp-metric | value]

10.end

DETAILED STEPS

Command or Action

Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configureterminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

routerbgpas-number

Example:

Device(config)# router bgp 40000

Enters router configuration mode for the specified routing process.

Step 4

address-familyipv4[unicast |
multicast |
vrfvrf-name]

Example:

Device(config-router)# address-family ipv4 unicast

Specifies the IPv4 address family and enters address family configuration mode.

Step 5

redistributeprotocolautonomous-system-number
route-map map-tag

Example:

Device(config-router-af)# redistribute bgp 100 route-map rtmap

Redistributes routes from one routing domain to another routing domain.

Step 6

networknetwork-idroute-map map-tag

Example:

Device(config-router-af)# network 10.1.1.1 route-map rtmap

Specifies the networks to be advertised by the Border Gateway Protocol (BGP) routing process.

Additional References for BGP Accumulated IGP

Related Documents

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Feature
Information for BGP Accumulated IGP

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

Table 1 Feature
Information for BGP Accumulated IGP

Feature
Name

Releases

Feature
Information

BGP
Accumulated IGP

15.4(2)S

The BGP
Accumulated IGP feature is an optional nontransitive Border Gateway Protocol
(BGP) path attribute. The attribute type code for the accumulated interior
gateway protocol (AIGP) attribute is assigned by the IANA. The value field of
the AIGP attribute is defined as a set of type, length, value (TLV) elements.
The AIGP TLV contains the AIGP metric.